The present invention relates to a PWM-pulse control system.
Specifically this invention relates, particularly but not exclusively, to a PWM-pulse control system for electric motors.
As is well known in this technical field, PWM (Pulse-Width Modulation) control signals are used in a large variety of electronic systems. In particular, they are used in DC/AC converters, PWM inverters to drive AC motors, and PLL (Phase-Locked Loops) control systems.
Recent developments in DC/AC converters have allowed the use of asynchronous motors to be extended to a broader range of industrial applications. In the past, asynchronous motors were only available for operation at frequency and voltage ratings set by their manufacturers.
In order to generate appropriate drive signals to an asynchronous motor at varying voltage and frequency, modulation techniques are required that utilize PWM signals. An example of such techniques is described in the Applicant""s U.S. Pat. No. 5,914,984. However, that technique is useful only in cases where voltage and current are phased to each other.
Furthermore, driving universal motors powered by the standard 220V AC power supply reference has a major problem in that the power input must be regulated to:
minimize electromagnetic emissions;
minimize power consumption; and
minimize the so-called output torque ripple.
More particularly, electromagnetic emissions from electric motors are to comply with international standards (e.g., European Standard EN 55014) established by survey of the emission spectra of signals at the terminals of the universal motor. The ideal is a zero-power emission spectrum at all frequencies, only showing a peak at 50 Hz, i.e. at the frequency of the AC power supply reference. Alternatively, the peak could locate at 0 Hz if a DC power supply is used.
It should be further considered that the amount of power dissipated through the electronic control of universal motors is made trivial, compared to that absorbed through the motor, by resorting to phase clipping and PWM modulation.
Conventional solutions to the problem of regulating the input power to a universal motor provide for:
amplitude modulation, using a potentiometer;
phase clipping through a triac; and
width modulation of a rectified-wave PWM signal.
An amplitude modulation controller using a potentiometer is shown schematically at 1 in FIG. 1. The controller 1 is connected across the terminals M1 and M2 of a universal motor 2 and is powered by a generator 3. In particular, the generator 3 may be the power supply reference at 220V. The controller 1 basically comprises a potentiometer having an equivalent load resistance R.
This amplitude modulation approach using a potentiometer represents a substantially perfect solution to the problem of keeping electromagnetic emission low. The power spectrum of the signal across the load comprising the motor 2 shows a peak at the 50 Hz frequency of the power supply 3 and nothing else.
Unfortunately, this is also the approach that involves maximum power dissipation. In particular, with the motor 2 stopped, the power dissipated is:
Pdiss=R*(I{circumflex over ( )}2) where,
Pdiss is the power dissipated,
R is the equivalent resistance of the potentiometer 1, and
I is the current flowing through the potentiometer 1.
On the other hand, with the motor 2 at full speed, the resistance RL of the universal motor is far below the resistance R of the potentiometer 1, and all the power is actually used up by the motor 2.
In any intermediate range of operation, some of the power is dissipated through the resistance R of the potentiometer 1, thereby raising power consumption to an unacceptably high level. It is for this reason that amplitude modulation is not widely made use of in actual practice to regulate the input power to universal electric motors.
The second of the above-listed solutions provides a controller with phase modulation using a triac, as shown schematically in FIG. 2. This controller, generally designated 4, is connected to one terminal M2 of a universal motor 2 and to a ground reference GND of a power supply reference 3.
The controller 4 comprises essentially a triac, having a control terminal TC4 connected to an external microcontroller MCU, not shown because conventional.
This would be the ideal approach from the standpoint of power consumption, were it not for a number of disadvantages, among which:
the output torque of the motor is not constant and shows a considerable torque ripple, especially at medium powers; as a result, the motor shaft is more heavily stressed than in other PWM approaches, so that the motor becomes very noisy and less reliable the time being;
the power spectrum of the signal across the electric motor is deeply affected at all frequencies by electric noise impossible to suppress even with sophisticated filters.
FIGS. 3A and 3B respectively show a normalized time signal across the motor plotted against time, and a power spectrum plotted against frequency for a phase modulation controller using a triac as described in relation to FIG. 2.
The third of the above-listed solutions provides for power modulation controllers using a PWM pulse, as shown in FIG. 4.
In particular, a controller of this type, designated 5, is connected between one terminal M1 of a universal motor 2 and a power supply reference 3, and comprises basically a diode bridge 6 having two input terminals T1, T3 and two output terminals T2, T4.
In addition, the controller 5 comprises a capacitor CAP connected in parallel between the output terminals T2, T4 of the diode bridge 6.
The second, T4, of said output terminals is connected to a ground reference GND, while the input terminals T1 and T3 of the diode bridge 6 are connected to the power supply 3.
The controller 5 further comprises an output power transistor Q connected between the other terminal M2 of the motor 2 and said ground reference GND. This transistor Q has a control terminal TC5 connected to a control output of an external microcontroller MCU, not shown because conventional.
The operation of this PWM controller 5 will now be described. The power supply 3 is rectified through the diode bridge 6 and converted to a DC signal through the capacitor CAP, which is here a large electrolytic capacitor. Only at this point, the PWM modulation is applied.
Therefore, power consumption through the controller 5 is fairly low, since all of the power from the power supply 3 goes into the motor 2. However, periodical charge/discharge cycling of the capacitor introduces a time variation in the rectified voltage across the load, as shown in FIG. 5A.
Thus, a signal is obtained that has a power spectrum with a large DC component, but significant noise at low frequencies, as can be seen from the log scale spectrum in FIG. 5B.
Using the electrolytic capacitor CAP also leads to increased circuit area requirements, while representing a critical factor in a high-temperature environment.
An embodiment of this invention provide a power regulating and modulating system for the power supplied to electric motors, preferably by means of PWM signals, which system has appropriate structural and functional features to operate without large capacitors, thereby overcoming the limitations of prior control systems and controllers.
One of the concepts behind embodiments of this invention is to provide a control system with power modulation of an unrectified PWM pulse, the load being connected between the power supply and the rectifier.